In the printing industry, halftone dot screen printing is used as a technique for creating images with varying levels of gray and color saturation and with varying color shades. With this technique, patterns of closely spaced tiny dots of ink of the appropriate color, or black, are deposited on paper or other printing surface to achieve the desired halftone screen. The dots are small enough and closely spaced enough as to be seen by the human eye at normal viewing distance as a continuous tone image when, in fact, it is a discontinuous image made up of thousands of dots. The varying levels of gray scale or color saturation are achieved by varying the sizes of the dots appropriately throughout the image, while the varying color shades are produced by superimposing the color and gray scale screens; the relative sizes of each color and black dot determining the composite shade of color or gray scale level.
One technique for electronically creating similar halftone images with varying dot sizes involves the subdivision of each dot into a matrix of pixel areas and to "build" a dot of the desired size by activating the appropriate number of pixels in the dot. An example of this is shown in FIGS. 1 and 2 wherein dot areas 10 are individually subdivided into a 12.times.12 matrix of pixel areas 11. In the illustrated example, a halftone image screen with 145 levels of gray or color saturation can be achieved either by leaving the dots blank or by activating from 1-144 of the pixels in each dot. The size of the dots and the number of pixel areas per dot is a matter of choice. Dot size determines the resolution or fineness of detail in the resulting image while the number of pixel areas in each dot determines the number of levels of gray and color saturation achievable in the image screen.
Various types of electronic scanning apparatus are known for creating a halftone image of the type described. For example, a single laser beam can be scanned repetitively by a line scanning mechanism across a target surface, which may be a photosensitive sheet, along horizontal rows or scan lines 12. The scan lines 12 are repeated progressively down the target surface in the cross-scan direction, either by deflecting the beam down by means of a galvano mirror or by moving the sheet up, thus creating a raster scan of the beam across the target surface. As the beam scans, digitized data signals representing the desired image screen are applied to turn the beam on and off thereby activating individual pixels 11a in each dot to create the desired pattern on the target surface. For definitional purposes, the resultant raster scanned image screen is described as having a spatial screen frequency given in dots per unit length which is equal to the reciprocal of the pitch between the centers of adjacent dots. Additionally, the spatial line scan frequency of the image screen is given in scan lines per unit length which is equal to the reciprocal of the pitch between the centers of vertically adjacent pixels. Line scan frequency can also be defined as the product of the number of pixels in the vertical direction (scan lines) times the screen frequency. Scanner apparatus contemplated by the present invention use multi-element line scanning devices to perform the raster scanning of the writing line across the target surface. Such devices may take the form of a multifaceted, rotating polygon mirror or a multifaceted, rotating holographic scan disk (hologon). With a single laser source, the lines are written one at a time. Alternatively, a plurality of scan lines can be written simultaneously with each horizontal traverse across the target surface by the line scanning device. Such a scanner may be a multi-laser source fiber optic scanner in which a vertical row of optical fibers moves plural writing beams across the target surface and the lines are written in parallel by suitably programmed image signals. Another example is an ink jet printer in which ink is squirted through a vertical row of apertures in a moving print head to deposit pixel sized ink dots on the target surface.
It has been found that the use of raster scanning as described above to generate halftone dot image screens can produce artifacts in the image referred to as moire banding which are perceived as horizontal bands of intensity variation in the image. The cause of moire banding lies in element-to-element nonuniformities in the line scanning device. These nonuniformities inject perturbations in the scanning beam at a repetition rate or frequency, as the raster scan progresses down the image screen, that beat with the halftone dot screen frequency to produce beat frequency perturbations that, in some cases, are visible as horizontal bands of intensity variation in the image. The spatial frequency and intensity of the moire effect is dependent on the number and magnitude of the nonuniformities in the scanner. Although production of a completely uniform scanner will eliminate moire banding, the expense and difficulty of producing such a device makes it impractical to rely on this as a solution to the problem of moire banding.
It is therefore an object of the present invention to provide scanning apparatus for generating halftone dot image screens that eliminates moire banding as a visible artifact in the resultant image.
It is a further object of the invention to provide scanning apparatus of the type described that makes moire banding artifacts in the image imperceptible without incurring the cost and difficulty involved in making completely uniform scanning devices such as would be required to eliminate moire banding artifacts entirely.
It is yet another object of the invention to reduce the perceptibility of moire in scanning apparatus of a variety of different types of scanning apparatus, such as, for example, in polygon mirror scanners, hologon scanners, multi-source fiber optic scanners and ink jet scanner printers wherein multi- element line scanner pixel-writing devices are used to generate a raster scanned halftone dot image screen.